In olefin epoxidation, a feed containing an olefin and an oxygen source is contacted with a catalyst under epoxidation conditions. The olefin is reacted with oxygen to form an olefin oxide. A product mix results that contains olefin oxide and, typically, unreacted feed and combustion products.
The olefin oxide may be reacted with water to form a 1,2-diol, with an alcohol to form a 1,2-diol ether, or with an amine to form an alkanolamine. Thus, 1,2-diols, 1,2-diol ethers, and alkanolamines may be produced in a multi-step process initially comprising olefin epoxidation and then the conversion of the formed olefin oxide with water, an alcohol, or an amine.
Olefin epoxidation catalysts comprise a silver component, usually with one or more additional elements deposited therewith, on a carrier. The presence of certain species contained in the carrier of the catalyst can be detrimental to the deposition process of catalytic species and/or catalyst performance. Various patents have focused on the pretreatment of carriers to improve the performance of a catalyst.
For example, U.S. Pat. No. 6,368,998-B1 shows that lowering the concentration of one or more ionizable species on the surface of a carrier and/or in the one or more materials used to make the carrier improves the performance of a catalyst in an epoxidation process. The concentration of ionizable species may be lowered by removing the species, rendering the species insoluble, or rendering the species immobile. Removal of the ionizable species may include contacting the carrier with de-ionized water as well as aqueous and/or organic solvent-based solutions containing tetraethylammonium hydroxide, ammonium acetate, lithium carbonate, barium acetate, strontium acetate, crown ether, methanol, ethanol, dimethylformamide, and mixtures thereof.U.S. Pat. No. 6,750,173-B2 shows that pretreatment of a carrier to remove sodium and partially replace the sodium with lithium improves the stability of a catalyst made from such carrier in an epoxidation process.
The catalyst performance may be assessed on the basis of selectivity, activity and stability of operation. The selectivity is the fraction of the converted olefin yielding the desired olefin oxide. As the catalyst ages, the fraction of the olefin converted normally decreases with time and to maintain a constant level of olefin oxide production (e.g., work rate) the temperature of the reaction may be increased. However, this adversely affects the selectivity of the conversion to the desired olefin oxide. In addition, the equipment used can tolerate temperatures only up to a certain level so that it is necessary to terminate the reaction when the reaction temperature reaches a level inappropriate for the reactor. Thus the longer the activity and selectivity can be maintained at acceptable values, the longer the catalyst charge can be kept in the reactor and the more product is obtained. Quite modest improvements in the selectivity and maintenance of the selectivity and activity over long periods yield substantial dividends in terms of process efficiency.